1. Field of the Invention
The present invention concerns a monolithic accelerometric transducer comprising a fixed part, a mobile mass part and a resonator having one of its two ends fastened to the mobile mass part.
The transducer is intended for a differential output accelerometer, for example. In particular, the invention provides a miniature accelerometer of low cost that can be used for navigation in aircraft and helicopters, for guiding missiles and for the active suspension of terrestrial vehicles, for example.
The resonator constituting the responsive member of the transducer of the invention is preferably a flexional or torsional vibratory blade of a piezoelectric material. The vibration frequencies of the blade are highly sensitive to the tensile or compression force exerted longitudinally on the blade when the mobile mass part constituting the inertial mass is subjected to acceleration. The extension or the compression of the blade is converted into electrical signals that are picked up by electrodes supported by the vibratory blade and connected to an oscillator circuit for example. A signal is produced at the output of the oscillator circuit, the frequency variations of the signal being representative of those of the acceleration.
Compared to conventional accelerometric sensors with an analog output (for example an electrical voltage), accelerometers with a frequency output have the advantage of potentially greater performance since frequency is readily processed in digital form.
Another important aspect is the monolithic nature of the transducer, which enables miniature accelerometers to be fabricated at relatively low cost by chemical machining and which promotes good performance since the process of assembling together component parts generally constitutes a major limitation of non-monolithic transducers.
The materials most frequently used to fabricate monolithic transducers are quartz and silicon, which are appreciated for the excellent stability of their mechanical characteristics.
2. Description of the Prior Art
As described in French patent application No. 2,685,964 in the name of the applicant, the body of an accelerometer CA' shown in FIG. 1 is monolithic and is obtained by chemical machining of piezoelectric crystals such as quartz. The body of the accelerometer CA' is generally parallelepiped shape and comprises a fixed mass part 1 fastened to a base ba, a mobile mass part 2, two vibrating blades 3.sub.1 and 3.sub.2 and two flexible hinges 44.sub.1 and 44.sub.2. The accelerometer body CA' is symmetrical about the central longitudinal axis z'z.
The blade 3.sub.1 is a beam having a small rectangular cross-section and flexional vibration of which is excited piezoelectrically by two metal electrodes 34.sub.1 and 34.sub.2 of opposite polarities. These electrodes are printed by a photolithographic process onto the external longitudinal side of the blade and terminate on the corresponding face F1 of the fixed part 1 in two conductive plates 33.sub.1 and 33.sub.2 connected to two first terminals of an oscillator circuit 5.sub.1 by means of two conductor wires 39.sub.1 and 39.sub.2. An identical arrangement of electrodes and plates is provided on the blade 3.sub.2 and the opposite face F2 of the fixed part 1, in relation to a second oscillator circuit 5.sub.2.
The outputs of the two oscillator circuits 5.sub.1 and 5.sub.2 are connected to a differential frequency measuring device including a frequency subtractor circuit 6 and a frequency meter 7, the frequency (f1-f2) measured by the frequency meter 7 being representative of the acceleration to be measured.
With reference to the mechanical design of the prior art accelerometer CA', the two hinges 44.sub.1 and 44.sub.2 are flexible in the sensitive direction DS perpendicular to the mid-plane pm of the body. If the accelerometer is subject to an acceleration in this sensitive direction DS the blades 3.sub.1 and 3.sub.2 are extended and compressed, i.e. they are subject to axial forces in opposite directions and proportional to the acceleration. This causes frequency variations of opposite signs for the two blades and, if the two blades are identical, frequency variations of the same magnitude. On the other hand, spurious input magnitudes such as the temperature generally have common mode effects on the two blades and cause frequency variations of the same sign. The benefit of the differential output (f.sub.1 -f.sub.2) is that it attenuates input magnitudes other than the acceleration in the sensitive direction DS.
The particular staircase shape of the hinges 44.sub.1 and 44.sub.2 allows chemical machining of the body CA' in a single step with the same depth of machining from the two faces F1 and F2 of the body of the transducer parallel to the plate pm.
The mechanical design of the prior art accelerometer CA' has disadvantages, in particular with regard to vibration of the two blades 3.sub.1 and 3.sub.2. First of all, mechanical loads such as the shear force and the bending moment where the two blades 3.sub.1 and 3.sub.2 are "built into" the fixed mass part 1 and generated by the vibrations of the two blades cause leakage of vibratory mechanical energy towards the base ba attached to it. This reduces the quality factor Q of the vibration of each of the blades 3.sub.1 and 3.sub.2. Secondly, the mechanical loads where these blades are "built into" the mobile part 2 and generated by their vibration cause small vibrational displacements of said mobile part at the same frequency as the vibrations of the blades 3.sub.1 and 3.sub.2. This results in mechanical coupling between the vibrations of the two blades, which disturbs their vibration. These two drawbacks affect the accuracy of the measured differential frequency (f1-f2) and therefore the value of the acceleration deduced therefrom.
FIG. 2 shows a second monolithic transducer disclosed in patent application WO 89/10568. The body of this second transducer made by chemical machining of a silicon wafer comprises a fixed part 21, an inertial mass 22 and three flexional vibratory resonators (filaments) 23, 24 and 25 excited by thermo-mechanical means, for example using heater elements (not shown) obtained by ion implantation on each resonator. The sensitive direction of this second prior art transducer is perpendicular to the faces of the wafer. The output signal of the transducer is a function of a linear combination of the frequencies of the three resonators 23, 24 and 25 independent of accelerations perpendicular to the sensitive direction.
The major disadvantage of the second prior art transducer is mechanical coupling between the vibrations of the three resonators, which affects the accuracy of the transducer.
FIG. 3 shows the body of a third monolithic transducer obtained by chemical machining of a silicon plate as disclosed in U.K. patent application No. 2,162,314. The body of the third transducer comprises a supporting frame 11 having flexible thin portions 15, and a resonator in the form of a double ended tuning fork constituted of two filaments 12 vibrating in antiphase and two root portions 13 and 14 attached to the supporting frame. The root portion 14 is connected to a fixed part 16 of the supporting frame and the other root portion 13 is connected to a U-shape mobile part 17 of the supporting frame constituting the inertial mass. The fixed and mobile parts 16 and 17 are connected together by the flexible portions 15 constituting hinges.
A first disadvantage of the third prior art transducer concerns the inadequate confinement of the vibratory mechanical energy in the two filaments 12, given the low mass of the mobile part 17.
A second disadvantage of the third prior art transducer concerns the location of the hinges 15 of the inertial mass 17, which is prejudicial to the thermo-mechanical behavior of the transducer when it is subjected to rapid variations in temperature. In this case, a difference appears between the temperature .theta..sub.1 of the fixed part 16 and the temperature .theta..sub.2 of the inertial mass 17, because these two parts are connected by the resonator 12 and the hinges 15, which can be regarded as thermal filters because of their small cross-section. The mean temperature of the resonator 12 is then substantially equal to that of the hinges 15 and is substantially equal to (.theta..sub.1 +.theta..sub.2)/2. The position of the hinges 15, parallel to the filaments 12, is substantially vertically in line with the root portion 14 of the resonator and accordingly the parallel branches of the U-shape inertial mass 17 extend substantially the entire length of the resonator. Accordingly, thermal expansion of the resonator and of the supporting frame 11 are not balanced and induce tensile or compression mechanical stresses in the resonator with a variation in frequency that is incorrectly interpreted as an acceleration.
FIG. 4 shows a prior art resonator for a thermostatically controlled oscillator featuring low power consumption and rapid heating as described in French patent application No. 2,688,954. Unlike the prior art acceleration transducers described previously, this resonator is designed to deliver a signal having a frequency that must be as stable as possible and which must therefore be relatively insensitive to acceleration. Thus the functions of the resonator are different from those of the transducer of the present invention.
From the structural point of view, the resonator shown in FIG. 4 comprises a central part R1 and a peripheral part R2 forming a ring surrounding the central part at a small radial distance and joined to the latter by an intermediate part R4 wherein an opening is formed. The peripheral part R2 is connected to the central part R1 by a single connecting bridge R3 which is constituted by the solid part of the intermediate part R4 and which extends over a small fraction of this intermediate part R4.
The central part R1 constitutes the active vibratory part of the resonator and the peripheral part R2 is immobilized in a casing by attached fixing means R5 such as a clamp located in an area of the peripheral part R2 opposite the single connecting bridge R3 relative to the active central part R1.
This embodiment with a single connecting bridge channels conducted heat flux and provides good control of the temperature of the resonator, by means of a heater element R6 and temperature sensor R7 at the level of the connecting bridge R3. The active central part R1 vibrates in shear in the direction of the thickness at a frequency in the order of 10 MHz. The vibratory mechanical energy is confined in the central part by virtue of the convex shape of at least one of the two major faces of the central part.